1. Field of the Invention
This invention relates to apparatus for annealing cylindrical vessels in situ by applying heat to the internal surface of the vessel, and more particularly, it applies to apparatus which is adjustable to accommodate vessels of various diameters and depths and which incorporates extendable and retractable thermocouple devices for monitoring vessel temperature during the annealing process.
2. Prior Art
Large pressure vessels such as those used to contain the cores of nuclear reactors are fabricated from steel plate sections which are welded together. These vessels are generally right cylindrical containers with an integral convex bottom, a radial flange around the top to which a separate cover is bolted, and inlet and outlet nozzles welded to openings in the cylindrical sides walls. Stresses created during fabrication are relieved by an annealing process carried out in a large oven.
It has been discovered that after prolonged exposure to the gamma rays produced by operation of the reactor, the vessel welds can become brittle. It has also been determined that annealing will restore the ductility of these welds, however, in order to avoid the creation of additional stresses, the entire vessel, not just the welds, must be brought to the annealing temperature. While this could be accomplished by placing the vessel in an annealing oven as during manufacture, it is just not practical to remove the now radioactive vessel from the ship or containment in which it is installed for annealing in an oven. It is preferable to anneal the vessel in situ, although this creates its own set of difficulties. Foremost among these is the fact that, since the surroundings in which the reactor vessel is installed, whether in a ship or a fixed installation, restrict access to the outside of the vessel, the heat for in situ annealing of the vessel must be applied solely from the interior of the vessel.
In one installation in which in situ annealing of a reactor vessel was carried out, a fixture having the general configuration of the cylindrical side walls and convex bottom of the vessel was suspended in the vessel. This fixture included axially adjacent rings extending along the cylindrical portion of the vessel each divided into four arcuate panels and a convex panel forming the bottom section. Each panel supported electric resistance heater wires arranged in a pattern to create a heating zone which provided a uniform watt density over the adjacent portion of the interior surface of the vessel. At the thicker portions of the vessel, such as at the flange along the upper edge and at the nozzles, the watt density was increased so that the entire vessel could be brought to a uniform annealing temperature, in the neighborhood of 1000.degree. Fahrenheit.
The watt density generated by each resistance heater was controlled by a system which included termocouples to measure the actual temperature of the vessel at each heating zone. Reliable measurement of the vessel temperature required that the junctions of the termocouples be pressed against the interior wall of the vessel with a predetermined force. Once the fixture was lowered into position, thermocouples were extended to contact the vessel wall by pushing them through conduit extending vertically downward, and for the side walls, curving radially outward. Some difficulty was encountered in applying the desired force to the thermocouples bearing against the side walls due to friction and binding of the thermocouple lead in the conduit.
While this apparatus was satisfactory for annealing vessels of the same size, reactor vessels vary in diameter and depth. A primary object of the present invention is to provide apparatus which can be easily adjusted for annealing a series of vessels having a range of sizes and to provide means for maintaining the thermocouples in contact with the vessel walls with a predictable preset force so that accurate temperature measurements can be taken during the annealing process.